The present invention relates to object examining systems, and to methods of operating such systems. The invention is particularly applicable to imaging objects by digital fluorography, and is therefore described below with respect to such application.
Digital fluorography systems are commonly operated according to either a dynamic mode for fast moving objects, or according to a static mode for stationary or slow moving objects.
In the dynamic mode, the X ray exposure is usually continuous, and the readout is usually interlaced to avoid flicker. The interlaced system used in the U.S.A. is a 525 line system, producing 30 frames/second, each frame including two interlaced fields; the interval for each field is approximately 14.5 ms, and each field is separated by blanking pulses of approximately 11/3 ms during the retrace intervals. The European system is a 625 line system with the blanking pulses during the retrace intervals being about 1.6 ms. In both systems, the initial images are of limited utility because at least one frame, and usually several frames, are required to allow the video signal level to stabilize. Thus, while this dynamic mode can image fast moving objects at a frame rate of 25 or 30 frames per second without flicker, it has the disadvantage of limited contrast because of the limited dosage permitted by the continuous X ray exposure.
The static mode is commonly used for imaging non moving or slow moving objects wherein a dynamic presentation at 25 or 30 frames per second is not required. In this mode, the X-ray source is periodically pulsed to expose the object, and the readout is usually a progressive one rather than an interlaced one. This is because an interlaced readout has smaller dose efficiency in view of the unavailability of the initial frames, and also lacks flexibiity in the choice of exposure time in view of the constraints of the X ray exposures to multiples of the TV frame time. Thus, while the pulse mode permits greater dosage, its main disadvantage is that it is useful only in non moving or slow moving objects as mentioned above.
An object of the present invention is to provide a new method and apparatus for examining objects, and particularly for imaging objects of digital fluorography, which new method and apparatus enables attaining higher frame rates than the conventional pulsed (static) mode, while at the same time enabling higher dosage, and therefore greater contrast, than the conventional continuous (dynamic) mode.